A spark plug is attached to an engine by screwing a male screw of a metallic shell of the spark plug into a threaded hole of the engine. Japanese Patent Application Laid-Open (kokai) No. 2007-280942 discloses a technique of preventing breakage of the metallic shell due to application of excessively large tightening torque. According to the disclosed technique, after formation of a male screw on a tubular member which is to become the metallic shell, the tubular member is carburized or quenched, so that the screw portion of the metallic shell is hardened over the entire circumference thereof.
The above-described conventional technique can increase the breaking load of the metallic shell through hardening of the screw portion. However, since the hardened screw portion has lowered toughness, when the speed of tightening (screwing) is high, the metallic shell may break with a load smaller than the breaking load in the case where the tightening speed is low.